1 Field of the Invention
The present invention relates to an image transmitting device for use in an electrophotographic copying machine, image reading machine, or the like and, more particularly, it relates to a control system for controlling the reciprocation of a scanning apparatus, wherein the scanning apparatus scans an original document in a programmed manner so as to sequentially transmit images of the original document in slits onto a photoreceptor surface.
2 Description of the Prior Art
Generally, an electrophotographic copying machine or an image reading machine has a platen or a support for holding an original document, and a scanning apparatus for scanning across the original document in slits. The scanning apparatus includes a source of light producing a light beams in the form of a ribbon that impinges on the original document and an image transmitting device for transmitting the image, i.e., the reflected light beam from the original document to the photoreceptor surface. The image transmitting device is defined by a pair of mirror devices, or a movable lens device (such as disclosed in U.S. Pat. No. 3584950 to Gundlach), and a driving device for driving such a transmitting device. When the transmitting device is defined by two mirror devices, the driving device moves the mirror devices at predetermined different speeds. For example, the two mirror devices move at speeds having a ratio of 2 to 1, thereby effecting a complete scan of the original document and, thereafter, returning the scanning apparatus back to its initial position.
According to the prior art, the scanning apparatus is coupled to a main motor through a suitable clutch device which may take one of three different positions: a neutral position; a forward transmitting position; and a reverse transmitting position. When the clutch device is in the neutral position, no driving force is transmitted to the scanning apparatus.
When the clutch device is in the forward transmitting position, the scanning apparatus is moved at a constant speed for effecting a scanning operation. More specifically, when the clutch device is shifted from the neutral position to the forward transmitting position, the scanning apparatus starts to move from its initial or home position, and its speed is accelerated until it reaches a predetermined constant speed. The acceleration of the scanning apparatus is carried out along a predetermined distance, hereinafter referred to as a pre-running distance. During the acceleration, the speed of the scanning apparatus is not constant, thus it is not suitable to carry out the image transmission during this period. When the scanning apparatus moves past the pre-running distance, it is moved at a constant speed for carrying out the image transmission from the original document to the receiving surface.
When the scanning arrangement completes one scan, the clutch device is shifted to the reverse transmitting position, thereby returning the scanning arrangement back to its initial position as quickly as possible for the next scanning operation.
In recent years many approaches have been made to improve the copying machine, such as to speed up the copying operation, to make the copy as precise as the original, and to reduce the size and complexity of the copying machine. To accomplish such approaches, it is necessary to provide an improved scanning apparatus that can scan quickly with high stability and high precision, and that returns back to the initial position with little deviation. Furthermore, it is desirable to make the pre-running distance as short as possible.
To speed up the copying operation, the returning speed is made much faster than the scanning speed. But, when this is done, it is necessary to give a braking effect in a highly controlled manner, otherwise, the scanning apparatus stops at a position greatly different from its home position. If the scanning apparatus fails to return precisely to its initial position, it is necessary to provide means for setting the scanning apparatus to the home position, or to provide a wide home position so that the scanning apparatus has a pre-running distance sufficiently long to accelerate the scanning apparatus to the required speed even when the scanning apparatus is stopped at a position greatly displaced towards the scanning area. The employment of the setting means or the long pre-running distance results in an increase in the number of parts or the size of the copying machine. Furthermore, it fails to speed up the copying operation.
To overcome the above disadvantages, various improved systems have been proposed, such as employing a servo-control system having a pulse motor or a D.C. motor. Such improvements are disclosed in, for example: U.S. Pat. No. 4,332,461 to Cail et al.; U.S. Pat. No. 4,287,461 to Promis et al.; and U.S. Pat. No. 4,371,254 to Beery.
In the proposed systems, however, a memory means is provided which stores information of proper speed and position of the scanning apparatus throughout the operation, and a servo-control that controls the actual speed and position to be the same as those stored in the memory means. The system according to Beery has a control means that controls only the return movement of the scanning arrangement, wherein the speed and position are controlled by the use of an optical grating.
The prior art systems are disadvantages in that a memory means having a large capacity must be used, and the design is complex.
In order to accomplish the return of scanning arrangement back to the home position with a simple structure, an improved control system for the scanning arrangement is disclosed in U.S. Pat. application Ser. No. 511,390, filed July 5, 1983, which is assigned to the same assignee as the present application. According to this U.S. Pat. application, a brake switch is provided in the path of the scanning apparatus so that during the return mode of the scanning apparatus, the scanning apparatus is first moved with full power when the apparatus hits the brake switch, it is braked to slow down to a certain low speed. Thereafter, the scanning apparatus is moved in the constant slow speed until it is returned back to the home position. In this manner, regardless of the scanned speed or distance, the scanning apparatus can be returned exactly back to the home position without constantly making a comparison between the preferred return pattern and the actual return pattern. Therefore, it is not necessary to provide a memory means having a large capacity. Thus, the control of the scanning apparatus can be accomplished with a simple structure.
However, according to this system, the distance for the scanning apparatus to return under the low constant speed becomes long, as the scan distance decreases, resulting in long period of time to complete one scan-and-return movement. The return distance under the low constant speed can be reduced by adding a timer which starts counting a preselected time in response to the actuation of the brake switch. When it counts the preselected time, the scanning apparatus starts braking. However, because the timer has some tolerance, and because of the deterioration with age of the timer and its associated parts, or variations with environmental changes, it is almost impossible to make the low speed return distance very short.